Purification of normal hydrocarbons by adsorption



Patented May 23, 19%? 3,321,396 PURIFICATION OF NORMAL HYDROCARBQNS BYABSORPTION Stanley Raymond Coilins Dryer, Feitham, John Edward Adams,Wokingham, and John Neil Turnbull, Twiclrenham, England, assignors toThe British Petroleum Company Limited, London, England, a corporation ofEngland No Drawing. Filed Aug. 5, 1964, Ser. No. 38%,982 Claimspriority, application Great Britain, Aug. 16, 1963,

8 Claims. (Cl. 208-310) This invention relates to the treatment ofn-paraffin fractions and particularly to n-parafiin fractions obtainedby the molecular sieve extraction of petroleum feedstocks.

It is well known that certain natural and synthetic zeolites have theproperty of preferentially absorbing certain types of hydrocarbons.These zeolites, known as molecular sieves, have crystalline structurescontaining a large number of pores of uniform size. In differentzeolites, these pores may vary from 4 A. to 15 A. or more in diameter,but in any one zeolite the pores will be of substantially uniform size.

It has previously been proposed to treat petroleum .fractions rangingfrom gasoline to gas-oils and higher with molecular sieves having porediameters ranging from 4 A. to 15 A. In order to separate straight chainhydrocarbons from branched chain and/or cyclic hydrocarbons a molecularsieve having pore diameters of 5 A. is suitable. Such a process may beused to recover a denormalised fraction, for example gasoline of higheroctane number due to the removal of low octane normal parafiins. Theabsorbed straight chain material may also be recovered as a product.

One important use for n-paraffins is as feedstocks for the preparationof the class of compounds known as measamolls. Mesamolls may be broadlydefined as esters having the general formula R'SO OR" where R is asaturated open chain hydrocarbon radical and R" is an aromatic oraliphatic radical. R preferably contains 920 carbon atoms per moleculeand n-parafiins in the C -C range are particularly suitable feedstocksfrom -which to prepare mesamolls, especially n-paraffins in this rangeobtained by the molecular sieve extraction of petroleum fractions.Mesamolls are useful as plasticizers and it is important that themesamoll should be as free from impurities as possible.

It has been found that the presence of even small quantities ofaromatics as impurity in the n-parafiin feedstock can have seriouseffects on the colour of the resulting mesamoll. It is thereforedesirable to reduce the mo maticcontent of the n-parafiin feedstock toas low a value as possible before preparing the mesamolls. The maximumtoleration of aromatics cannot be stated with certainty since this valueis below that to which current analytical techniques for measuringaromatic content can be accurately applied. It is believed, however,that the aromatic content should be reduced to below 0.01%. In practicea convenient method of assessing the stability of an n-paraffin fractionis to prepare from it a sample of a mesamoll in known manner and tomeasure the colour of the mesamoll on a Lovibond tintometer. A colour ofless than 2.0Y in a 2" cell is considered satisfactory. Other potentialuses for n-parafiins require a low aromatic content, for example as anoxidation feedstock where aromatics can act as powerful inhibitors ofthe process.

By careful control of the molecular sieve extraction process, n-parafiinfractions of greater than 95% Weight purity can be obtained but thesefractions still contain aromatics in an amount up to at least 2.5%weight. Some further treatment is therefore required in order to obtainn-parafiins suitable for mesamoll production.

According to the present invention a process for the preparation ofn-paraffin fractions from petroleum fractions comprises subjecting thepetroleum fraction to extraction with a 5 A. molecular sieve to separatean nparaflinic fraction, treating the n-parafiinic fraction with silicagel to adsorb aromatic hydrocarbons, displacing nparafiins held up inthe silica gel using liquid pentane at ambient temperature, desorbingaromatics from the silica gel using butane or pentane at elevatedtemperature in either the liquid or vapour phase, and reusing the silicagel to treat more n-parafiinic material.

By contacting the silica gel with butane or pentarie at ambienttemperature before the desorption step proper, any non-aromatic materialheld up in the silica gel may be removed from the system and passed withthe dearomatised n-parafiin stream to enhance the yield of n-paraffins.In addition, this intermediate step avoids any re-contamination ofn-paraflins with aromatics which might result if no such step wasemployed.

The desorption step may take place in either the vapour phase or liquidphase. Suitable vapour phase conditions include a temperature within therange 50400 C., a pressure within the range atmospheric-1000 p.s.i.g.,and a liquid space velocity within the range 0.1-5 v./v./hr. Suitableliquid phase conditions include a temperature within the range 15250 C.,a pressure within the range atmospheric-1000 p.s.i.g., and a spacevelocity within the range 0.1-5 v./ v./ hr.

The silica gel treatment is preferably carried out in a fixed bed.Preferably, in order to allow fully cyclic operation, at least two bedsof silica gel are provided so that while one bed is being desorbed,another may be adsorbing.

The process of the present invention is particularly suitable for theproduction of n-parafiins for mesamoll preparation. Preferably then-paraffinic fraction obtained from the molecular sieve extractioncontains at least of n-paraffins. A suitable molecular sieve extractionprocess which is capable of giving a 95% pure n-paraflin fractioncomprises a three stage, isothermal, vapour phase process in which apetroleum feedstock is contacted with a 5 A. molecular sieve in a firststage to absorb straight chain hydrocarbons, the sieve is contacted witha purging medium in a second stage to remove material adsorbed on thesurface of the sieve or held interstitially between sieve particles, andthe absorbed straight chain hydrocarbons are desorbed in a third stageusing n-pentane or nbutane as desorbing medium, the pressure in thedesorption stage being equal to, or greater than, the pressure in theadsorption stage.

Such a process for treating C C petroleum fractions is described, forexample, in our co-pending application Ser. No. 168,981, filed Ian. 26,1962, now US. Patent No. 3,201,490. Preferably, the petroleum fractionprior to molecular sieve extraction is subjected to a hydrocatalyticdesulphurisation step to reduce the sulphur content, preferably to avalue between 0.002 and 0.015% weight. Suitable petroleum feedstocks forthe process of the present invention lie within the range C and above,particularly C9-Cg0.

An important feature of the present invention is that desorption of thesilica gel may be achieved using the same desorbent as that employed todesorb the molecular sieve. The use of pentane is particularlypreferred. This mode of operation enables the overall process ofmolecular sieve extraction and silica gel treatment to be operated muchmore economically than if a ditferent desorbing medium was used todesorb the silica gel, thereby necessitating the provision of additionalstorage and recovery equipment.

If desired a final polishing treatment may be given to the silica geltreated product to clear up any trace im- 3 purities remaining. Such atreatment may comprise, for example, a bauxite treatment or aredistillation step.

The invention is illustrated by the following example:

Example A n-parafiinic fraction was obtained by the molecular sieveextraction of a petroleum fraction boiling within the range 200300 C. ina three stage adsorption-purgedesorption process carried out under theconditions indicated in the following Table 1:

A n-paraflinic product boiling within the range 200- 300 C. andcontaining 1.2% weight of aromatic hydrocarbons was obtained. Thisproduct was passed over a bed of silica gel for 4 hours at a spacevelocity of 0.64

w./w./hr., at ambient temperature (20 C.), and a pressure of 100p.s.i.g. Liquid pentane was then passed at ambient temperature (20 C.)over the silica gel at a space velocity of 0.52 v./v./hr. for 4 hours.The silica gel was finally desorbed for 8 hours at a space velocity of0.52 v./v./ hr. using hot pentane in the liquid and vapour phase atvarious temperatures and pressures. The results are given in thefollowing Table 2:

pentane at ambient temperature, desorbing aromatics from the silica gelusing butane or pentane at elevated temperature in either the liquid orvapour phase, and reusing the silica gel to treat more n-parafiinicmaterial.

2. A process as claimed in claim 1 wherein the aromatics are desorbedfrom the silica gel in the vapour phase at a temperature within therange 40-400 C., a pressure within the range atmospheric-1000 p.s.i.g.,and a liquid space velocity within the range 0.1-5 v./ v./ hr.

3. A process as claimed in claim 1 wherein the aromatics are desorbedfrom the silica gel in the liquid phase at a temperature within therange 250 C., a pressure within the range atmospheric-1000 -p.s.i.g.,and a space velocity within the range 0.1-5 v./v./hr.

4. A process as claimed in claim 1 wherein the silica gel treatment iscarried out in at least one fixed bed.

5. A process as claimed in claim 1 wherein the molecular sieveextraction is conducted so as to yield a n-paraffinic fractioncontaining at least 95% by weight of n-parafiins.

6. A process as claimed in claim 1 wherein the petroleum feedstock boilswithin the range C to C 7. A process as claimed in claim 1 wherein thedesorption of the silica gel is achieved using the same desorbent asthat employed to desorb the molecular sieve during the molecular sieveextraction stage, this desorbent preferably being n-pentane.

8. A process as claimed in claim 1 wherein the silica gel treatedproduct is given a final polishing treatment to remove any traceimpurities.

TABLE 2 Recoveries percent wt. (pen- Aromatics Recovery tane free)Feedstock Conditions of Desorption Wt. of Aroma- Wt. oi Aroma- BulkRecovery ties recovered ties recovered Total Recovery of N-paratl'insWt. 01' Aromain desorption in percentage of oil of Mesamoll ties, ted.lbs. pentane (by of maximum standard LB. or U.V. possible reanalysis),lbs. covery Vapour phase pentane at 500 F. and atmospheric pressure 9S.5 B4. 5 1.70 1. 40 82. 4 Vapour phase pentane at 700 F. and atmospheriepressure 97- 4 1. 59 1. 40 88. 1 Liquid phase peutane at ZOO-220 F. and115 p.s.i.g. pressure 96. 0 96. 0 1. 72 1:60 93. 02

It is seen that a good recovery of n-parafiins of mesa- References Citedby the Examiner moll standard was obtained. UNITED STATES PATENTS Weclaim: 1 A rocess for the pre aration of n-paraffin fractions 26327783/1953 Jonach 208310 fro e roleum fracions c m risin sub' ctin th etro2773803 12/1956 Van der Fear et 208-310 In P L v P 2,916,446 12/1959Shuman 208-310 leurn fractions to extraction with a 5 A. molecular sieve3,216,928 11/1965 Paulson 208*310 to separate a n-paraffinic fraction,treating the n-paraffinic fraction with silica gel to adsorb aromatichydrocarbons, displacing n-parafiins held up in the silica gel usingliquid DELBERT E. GANTZ, Primary Examiner.

A. D. SULLIVAN, Assistant Examiner.

1. A PROCESS FOR THE PREPARATION OF N-PARAFFIN FRACTIONS FROM PETROLEUMFRACTIONS COMPRISING SUBJECTING THE PETROLEUM FRACTIONS TO EXTRACTIONWITH A 5 A. MOLECULAR SIEVE TO SEPARATE A N-PARAFFINIC FRACTION,TREATING THE N-PARAFFNIC FRACTION WITH SILICA GEL TO ADSORB AROMATICHYDROCARBONS, DISPLACING N-PARAFFINS HELD UP IN THE SILICA GEL USINGLIQUID PENTANE AT AMBIENT TEMPERATURE, DESORBING AROMATICS FROM THESILICA GEL USING BUTANE OR PENTANE AT ELEVATED TEMPERATURE IN EITHER THELIQUID OR VAPOUR PHASE, AND REUSING THE SILICA GEL TO TREAT MOREN-PARAFFINIC MATERIAL.